Add option to serve Prometheus metrics #671 (#693)

Co-authored-by: Florent FAYOLLE <florent.fayolle@beta.gouv.fr>

stubs/app/server/prometheus-exporter.ts

@@ -0,0 +1,25 @@
+import { collectDefaultMetrics, register } from 'prom-client';
+import http from 'http';
+
+const reqListener = (req: http.IncomingMessage, res: http.ServerResponse) => {
+  register.metrics().then((metrics) => {
+    res.writeHead(200, { 'Content-Type': register.contentType });
+    res.end(metrics);
+  }).catch((e) => {
+    res.writeHead(500);
+    res.end(e.message);
+  });
+};
+
+export function runPrometheusExporter(port: number) {
+  collectDefaultMetrics();
+
+  if (isNaN(port)) {
+    throw new Error(`Invalid port: ${process.env.GRIST_PROMCLIENT_PORT}`);
+  }
+  const server = http.createServer(reqListener);
+  server.listen(port, '0.0.0.0');
+
+  console.log(`Prometheus exporter listening on port ${port}.`);
+  return server;
+}

stubs/app/server/server.ts

@@ -36,6 +36,7 @@ setDefaultEnv('GRIST_WIDGET_LIST_URL', commonUrls.gristLabsWidgetRepository);
 import {updateDb} from 'app/server/lib/dbUtils';
 import {main as mergedServerMain, parseServerTypes} from 'app/server/mergedServerMain';
 import * as fse from 'fs-extra';
+import {runPrometheusExporter} from './prometheus-exporter';
 
 const G = {
   port: parseInt(process.env.PORT!, 10) || 8484,
@@ -102,6 +103,10 @@ export async function main() {
     console.log('For full logs, re-run with DEBUG=1');
   }
 
+  if (process.env.GRIST_PROMCLIENT_PORT) {
+    runPrometheusExporter(parseInt(process.env.GRIST_PROMCLIENT_PORT, 10));
+  }
+
   // If SAML is not configured, there's no login system, so provide a default email address.
   setDefaultEnv('GRIST_DEFAULT_EMAIL', 'you@example.com');
   // Set directory for uploaded documents.